Such a turbocharger is represented in document U.S. Pat. No. 7,010,917 B2.
According to this document, it is provided for a force-fit connection between shaft and compressor wheel to axially tension the compressor wheel between a nut, which is screwed on a screw thread on the free end of the shaft, and an annular step on the shaft. In order to increase the transferable torque between shaft and compressor wheel, the contact surfaces, which are resting one atop the other, of the nut and compressor wheel are configured to increase friction. In order to enable the contact surfaces, which are configured as friction increasing, to be effective, high axial tension forces must be present. Therefore, plastic deformations must be anticipated in the connection region between shaft and compressor wheel, said plastic deformations limiting the transferable torque, because the turbine wheel that is impinged upon with hot exhaust gas causes a high heat flow over the shaft to the compressor wheel and therefore high temperatures can arise in the connection area between shaft and compressor wheel.
In document EP 0 961 038 A1, a connecting element for a friction-increasing connection of work pieces to be fused is described. This connecting element consists of a thin, resiliently elastic film that supports particles of a defined size on its surface, wherein these particles consist of a material having a pressure and shear resistance that surpasses that of the work pieces to be fused. In the force-fit connection of the work pieces to be fused, these particles are pressed into the work piece surfaces, which are positioned opposite one another, and thereby produce a form-fitting connection between the work pieces.
It is disadvantageous here that in the assembly of the force-fit connection, a comparably large number of parts must be handled and supplied. Moreover, an incorrect assembly with a connecting element is not readily recognisable, meaning that a high degree of monitoring outlay is necessary during the assembly process.
Furthermore, with the increase in the number of parts, the number increases of the dimensions in the tolerance chain of the parts to be assembled, resulting in the fact that enlarged air gaps, which can be disadvantageous, can arise on the turbine and/or compressor side of the turbocharger.
The object of the invention is now to create for a turbocharger of the previously mentioned type a frictional connection, which can be particularly highly stressed, between compressor wheel and rotor shaft.
This problem is solved according to the invention in that the torque-transferring adhered surfaces of the force-fitting connection are provided entirely or partially with a friction-increasing coating.